In a wireless communication system, a transmitter modulates a baseband signal with a high frequency carrier to transmit to a receiver. The modulated signal is received and demodulated at the receiver. For modulation and demodulation, both a transmitter and a receiver include a local oscillator for generating the same frequency carrier signal. To effectively demodulate the modulated signal, the frequency of the local oscillator at the receiver should be same as that of the transmitter. Therefore, it is important to remove phase errors between the transmitter and the receiver local oscillators.
Although the nominal frequencies of a wireless transmit/receive unit (WTRU) and a base station local oscillator are the same, they are different in practice. There are two main reasons for the difference. The first reason is an initial frequency error due to manufacturing tolerances of the oscillator. The second reason is a drift of the oscillator frequency over time. This happens due to various reasons such as temperature effects and aging. This cumulatively results in different oscillator frequencies between a WTRU and a base station.
The frequency difference of local oscillators between a WTRU and a base station causes system degradation. For example, in a Universal Terrestrial Radio Access/Time Division Duplex (UTRA/TDD) system, the frequency difference between a base station local oscillator and a WTRU local oscillator can be as large as +3 ppm. With a transmitter and receiver carrier frequency of about 2 GHz, 3 ppm corresponds to a frequency error of 6,000 Hz. Since the local oscillator is synthesized from the same local oscillator that is used for sampling, sampling at the receiver can drift as much as 1 chip every 8.7 frames for a TDD system.